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Snowball Earth
The Snowball Earth proposes that during one or more of Earth's climates, became entirely or nearly entirely frozen, sometime earlier than 650 (million years ago). Proponents of the hypothesis argue that it best explains deposits generally regarded as of origin at s and other enigmatic features in the record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the feasibility of an - or -covered ocean and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water. The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms, known as the . The most recent snowball episode may have triggered the evolution of multicellularity. Another, much earlier and longer snowball episode, the , which would have occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the " ". History Evidence for ancient glaciation mounts Long before the idea of a global glaciation was established, a series of discoveries began to accumulate evidence for ancient Precambrian glaciations. The first of these discoveries was published in 1871 by J. Thomson who found ancient glacier-reworked material ( ) in , Scotland. Similar findings followed in Australia (1884) and India (1887). A fourth and very illustrative finding that came to be known as " " was reported by in northern Norway in 1891. Many other findings followed, but their understanding was hampered by the rejection of . Global glaciation proposed Sir (1882–1958), an Australian geologist and Antarctic explorer, spent much of his career studying the of South Australia, where he identified thick and extensive glacial sediments and late in his career speculated about the possibility of global glaciation. Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and those of other continents where low-latitude glacial deposits are found, have remained constant through time. With the advancement of the hypothesis, and eventually theory, came an easier explanation for the glaciogenic sediments—they were deposited at a time when the continents were at higher latitudes. In 1964, the idea of global-scale glaciation reemerged when published a paper in which he presented data showing that glacial ites in and were deposited at tropical latitudes. From this palaeomagnetic data, and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued for an that was so extreme that it resulted in the deposition of marine glacial rocks in the tropics. In the 1960s, , a Russian climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global . Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions, a feedback loop ensued where the increased reflectiveness ( ) of the ice led to further cooling and the formation of more ice, until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium. While Budyko's model showed that this ice-albedo stability could happen, he concluded that it had in fact never happened, because his model offered no way to escape from such a feedback loop. In 1971, Aron Faegre, an American physicist, showed that a similar energy-balance model predicted three stable global climates, one of which was snowball earth. This model introduced 's concept of intransitivity indicating that there could be a major jump from one climate to another, including to snowball earth. The term "snowball Earth" was coined by in a short paper published in 1992 within a lengthy volume concerning the biology of the eon. The major contributions from this work were: (1) the recognition that the presence of s is consistent with such a global glacial episode, and (2) the introduction of a mechanism by which to escape from a completely ice-covered Earth—specifically, the accumulation of CO2 from volcanic outgassing leading to an ultra- . 's discovery of a consistent geological pattern in which lake levels rose and fell is now known as the "Van Houten cycle". His studies of phosphorus deposits and in sedimentary rocks made him an early adherent of the "snowball Earth" hypothesis postulating that the planet's surface froze more than 650 million years ago. Interest in the notion of a snowball Earth increased dramatically after and his co-workers applied Kirschvink's ideas to a succession of Neoproterozoic sedimentary rocks in and elaborated upon the hypothesis in the journal Science in 1998 by incorporating such observations as the occurrence of s. In 2010, Francis MacDonald reported evidence that was at equatorial latitude during the period with glacial ice at or below sea level, and that the associated was global. Evidence The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes. According to modelling, an would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25° to 30° of the equator. Therefore, the presence of glacial deposits within the suggests global ice cover. Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove two things: # that a bed contains sedimentary structures that could have been created only by glacial activity; # that the bed lay within the tropics when it was deposited. During a period of global glaciation, it must also be demonstrated that glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence. This last point is very difficult to prove. Before the , the markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at precisely the same time. The best that can be done is to estimate the age of the rocks using methods, which are rarely accurate to better than a million years or so. The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the approximate latitudes of landmasses even as recently as can be riddled with difficulties. Palaeomagnetism The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth's long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism. When s form, magnetic minerals within them tend to align themselves with the . Through the precise measurement of this , it is possible to estimate the (but not the ) where the rock matrix was formed. Palaeomagnetic measurements have indicated that some sediments of glacial origin in the rock record were deposited within 10 degrees of the equator, although the accuracy of this reconstruction is in question. This palaeomagnetic location of apparently glacial sediments (such as s) has been taken to suggest that glaciers extended from land to sea level in tropical latitudes at the time the sediments were deposited. It is not clear whether this implies a global glaciation, or the existence of localized, possibly land-locked, glacial regimes. Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator. Skeptics suggest that the palaeomagnetic data could be corrupted if Earth's ancient magnetic field was substantially different from today's. Depending on the rate of cooling of , it is possible that during the Proterozoic, the did not approximate a simple distribution, with north and south magnetic poles roughly aligning with the planet's axis as they do today. Instead, a hotter core may have circulated more vigorously and given rise to 4, 8 or more poles. Palaeomagnetic data would then have to be re-interpreted, as the sedimentary minerals could have aligned pointing to a 'West Pole' rather than the North Pole. Alternatively, Earth's dipolar field could have been oriented such that the poles were close to the equator. This hypothesis has been posited to explain the extraordinarily rapid motion of the magnetic poles implied by the Ediacaran palaeomagnetic record; the alleged motion of the north pole would occur around the same time as the Gaskiers glaciation. Another weakness of reliance on palaeomagnetic data is the difficulty in determining whether the magnetic signal recorded is original, or whether it has been reset by later activity. For example, a mountain-building releases hot water as a by-product of metamorphic reactions; this water can circulate to rocks thousands of kilometers away and reset their magnetic signature. This makes the authenticity of rocks older than a few million years difficult to determine without painstaking mineralogical observations. Moreover, further evidence is accumulating that large-scale remagnetization events have taken place which may necessitate revision of the estimated positions of the palaeomagnetic poles. There is currently only one deposit, the Elatina deposit of Australia, that was indubitably deposited at low latitudes; its depositional date is well-constrained, and the signal is demonstrably original. Low-latitude glacial deposits of the Pocatello Formation, a "snowball Earth"-type deposit}} below site in the , South Australia. for scale.}} Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the snowball Earth hypothesis many sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, it is worth remembering that many sedimentary features traditionally associated with glaciers can also be formed by other means. Thus the glacial origin of many of the key occurrences for snowball Earth has been contested. As of 2007, there was only one "very reliable"—still challenged—datum point identifying tropical s, which makes statements of equatorial ice cover somewhat presumptuous. However, evidence of sea-level glaciation in the tropics during the is accumulating. Evidence of possible glacial origin of sediment includes: * (stones dropped into marine sediments), which can be deposited by glaciers or other phenomena. * (annual sediment layers in periglacial lakes), which can form at higher temperatures. * s (formed by embedded rocks scraped against bedrock): similar striations are from time to time formed by s or tectonic movements. * s (poorly sorted conglomerates). Originally described as glacial , most were in fact formed by s. Open-water deposits It appears that some deposits formed during the snowball period could only have formed in the presence of an active hydrological cycle. Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers melted and re-formed repeatedly for tens of millions of years; solid oceans would not permit this scale of deposition. It is considered possible that s such as seen in today could have caused these sequences. Further, sedimentary features that could only form in open water (for example: , far-traveled and indicators of photosynthetic activity) can be found throughout sediments dating from the snowball-Earth periods. While these may represent "oases" of on a completely frozen Earth, computer modelling suggests that large areas of the ocean must have remained ice-free; arguing that a "hard" snowball is not plausible in terms of energy balance and general circulation models. Carbon isotope ratios There are two stable s of carbon in : (12C) and the rare (13C), which makes up about 1.109 percent of carbon atoms. Biochemical processes, of which is one, tend to preferentially incorporate the lighter 12C isotope. Thus ocean-dwelling photosynthesizers, both s and , tend to be very slightly depleted in 13C, relative to the abundance found in the primary sources of Earth's carbon. Therefore, an ocean with photosynthetic life will have a lower 13C/12C ratio within organic remains, and a higher ratio in corresponding ocean water. The organic component of the lithified sediments will remain very slightly, but measurably, depleted in 13C. During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of 13C to 12C. Close analysis of the timing of 13C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic. Banded iron formations (BIF) are sedimentary rocks of layered and iron-poor . In the presence of oxygen, naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of the Earth's atmosphere during the era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and precipitated out as iron oxide. The bands were produced at the between an and an oxygenated ocean. Since today's atmosphere is -rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with glacial deposits. For such iron-rich rocks to be deposited there would have to be anoxia in the ocean, so that much dissolved iron (as ) could accumulate before it met an oxidant that would precipitate it as oxide. For the ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea-ice, while opponents suggest that the rarity of the BIF deposits may indicate that they formed in inland seas. Being isolated from the oceans, such lakes could have been stagnant and anoxic at depth, much like today's ; a sufficient input of iron could provide the necessary conditions for BIF formation. A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments. BIFs are also strikingly absent during the . Cap carbonate rocks Around the top of glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary or metres to tens of metres thick. These cap carbonates sometimes occur in sedimentary successions that have no other carbonate rocks, suggesting that their deposition is result of a profound aberration in ocean chemistry. , possibly ending the global ice age of the Period.}} These s have unusual chemical composition, as well as strange sedimentary structures that are often interpreted as large ripples. The formation of such sedimentary rocks could be caused by a large influx of positively charged , as would be produced by rapid weathering during the extreme greenhouse following a snowball Earth event. The isotopic signature of the cap carbonates is near −5 ‰, consistent with the value of the mantle—such a low value is usually/could be taken to signify an absence of life, since photosynthesis usually acts to raise the value; alternatively the release of methane deposits could have lowered it from a higher value, and counterbalance the effects of photosynthesis. The precise mechanism involved in the formation of cap carbonates is not clear, but the most cited explanation suggests that at the melting of a snowball Earth, water would dissolve the abundant from the to form , which would fall as . This would weather exposed and (including readily attacked glacial debris), releasing large amounts of , which when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock. Such an " " sediment can be found on top of the glacial till that gave rise to the snowball Earth hypothesis. However, there are some problems with the designation of a glacial origin to cap carbonates. Firstly, the high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic, and dissolve any carbonates contained within—starkly at odds with the deposition of cap carbonates. Further, the thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin. An alternative mechanism, which may have produced the cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low—as low as −48 ‰— values—as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments. Changing acidity Isotopes of the element suggest that the of the oceans dropped dramatically before and after the glaciation. This may indicate a buildup of in the atmosphere, some of which would dissolve into the oceans to form . Although the boron variations may be evidence of extreme , they need not imply a global glaciation. Space dust Earth's surface is very depleted in the element , which primarily resides in the Earth's core. The only significant source of the element at the surface is that reach Earth. During a snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An has been discovered at the base of the cap carbonate formations, and has been used to suggest that the glacial episode lasted for at least 3 million years, but this does not necessarily imply a global extent to the glaciation; indeed, a similar anomaly could be explained by the impact of a large . Cyclic climate fluctuations Using the ratio of mobile s to those that remain in soils during (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods. This pattern, if a true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to cycles than to a completely frozen Earth. In addition, glacial sediments of the in Scotland clearly show interbedded cycles of glacial and shallow marine sediments. The significance of these deposits is highly reliant upon their dating. Glacial sediments are difficult to date, and the closest dated bed to the Portaskaig group is 8 km stratigraphically above the beds of interest. Its dating to 600 Ma means the beds can be tentatively correlated to the Sturtian glaciation, but they may represent the advance or retreat of a snowball Earth. Mechanisms The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase Earth's , which would result in for cooling. If enough snow and ice accumulates, run-away cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where is most direct. Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a , a reduction in the atmospheric concentration of es such as and/or , changes in , or perturbations of . Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more Solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen as cold as modern . associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium. Continental distribution A tropical distribution of the continents is, perhaps counter-intuitively, necessary to allow the initiation of a snowball Earth. Firstly, tropical continents are more reflective than open ocean, and so absorb less of the Sun's heat: most absorption of Solar energy on Earth today occurs in tropical oceans. Further, tropical continents are subject to more rainfall, which leads to increased river discharge—and erosion. When exposed to air, rocks undergo weathering reactions which remove carbon dioxide from the atmosphere. These reactions proceed in the general form: Rock-forming mineral + CO2 + H2O → cations + bicarbonate + SiO2. An example of such a reaction is the weathering of : : CaSiO3 + 2CO2 + H2O → Ca2+ + SiO2 + 2HCO3− The released cations react with the dissolved in the ocean to form as a chemically precipitated . This transfers , a greenhouse gas, from the air into the , and, in steady-state on geologic time scales, offsets the carbon dioxide emitted from es into the atmosphere. As of 2003, a precise continental distribution during the Neoproterozoic was difficult to establish because there were too few suitable sediments for analysis. Some reconstructions point towards polar continents—which have been a feature of all other major glaciations, providing a point upon which ice can nucleate. Changes in ocean circulation patterns may then have provided the trigger of snowball Earth. Additional factors that may have contributed to the onset of the Neoproterozoic snowball include the introduction of atmospheric free oxygen, which may have reached sufficient quantities to react with , oxidizing it to carbon dioxide, a much weaker greenhouse gas, and a younger—thus fainter—Sun, which would have emitted 6 percent less radiation in the Neoproterozoic. Normally, as Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation, the cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and Earth warms as this greenhouse gas accumulates—this ' ' process limits the magnitude of cooling. During the period, however, Earth's continents were all at latitudes, which made this moderating process less effective, as high weathering rates continued on land even as Earth cooled. This let ice advance beyond the polar regions. Once ice advanced to within 30° of the equator, a positive feedback could ensue such that the increased reflectiveness ( ) of the ice led to further cooling and the formation of more ice, until the whole Earth is ice-covered. Polar continents, due to low rates of , are too dry to allow substantial carbon deposition—restricting the amount of atmospheric carbon dioxide that can be removed from the . A gradual rise of the proportion of the carbon-13 relative to carbon-12 in sediments pre-dating "global" glaciation indicates that draw-down before snowball Earths was a slow and continuous process. The start of snowball Earths are always marked by a sharp downturn in the δ13C value of sediments, a hallmark that may be attributed to a crash in biological productivity as a result of the cold temperatures and ice-covered oceans. In January 2016, Gernon et al. proposed a "shallow-ridge hypothesis" involving the breakup of the , linking the eruption and rapid alteration of s along shallow ridges to massive increases in alkalinity in an ocean with thick ice cover. Gernon et al. demonstrated that the increase in alkalinity over the course of glaciation is sufficient to explain the thickness of cap carbonates formed in the aftermath of Snowball Earth events. During the frozen period Global temperature fell so low that the equator was as cold as modern-day . This low temperature was maintained by the high albedo of the ice sheets, which reflected most incoming solar energy into space. A lack of heat-retaining clouds, caused by water vapor freezing out of the atmosphere, amplified this effect. Breaking out of global glaciation The levels necessary to thaw Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere. Since the Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by release of alkaline metal ions weathering out of s. Over 4 to 30 million years, enough and , mainly emitted by es but also produced by microbes converting organic carbon trapped under the ice into the gas, would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of permanently ice-free land and water developed; this would be darker than the ice, and thus absorb more energy from the Sun—initiating a " ". Destabilization of substantial deposits of s locked up in low-latitude may also have acted as a trigger and/or strong positive feedback for deglaciation and warming. On the continents, the melting of s would release massive amounts of glacial deposit, which would erode and weather. The resulting sediments supplied to the ocean would be high in nutrients such as , which combined with the abundance of would trigger a population explosion, which would cause a relatively rapid reoxygenation of the atmosphere, which may have contributed to the rise of the and the subsequent —a higher oxygen concentration allowing large multicellular lifeforms to develop. Although the loop would melt the ice in geological short order, perhaps less than 1,000 years, replenishment of atmospheric oxygen and depletion of the levels would take further . It is possible that carbon dioxide levels fell enough for Earth to freeze again; this cycle may have repeated until the to more polar latitudes. More recent evidence suggests that with colder oceanic temperatures, the resulting higher ability of the oceans to dissolve gases led to the carbon content of sea water being more quickly oxidized to carbon dioxide. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of Earth's surface, and the prevention of a total snowball state. During millions of years, would have accumulated on and inside the ice. microorganisms, volcanic ash and dust from ice-free locations would settle on ice covering several million square kilometers. Once the ice started to melt, these layers would become visible and color the icy surfaces dark, helping to accelerate the process. Ultraviolet light from the Sun would also produce hydrogen peroxide (H2O2) when it hits water molecules. Normally hydrogen peroxide is broken down by sunlight, but some would have been trapped inside the ice. When the glaciers started to melt, it would have been released in both the ocean and the atmosphere, where it was split into water and oxygen molecules, leading to an increase in atmospheric oxygen. Slushball Earth hypothesis While the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to posit a "slushball Earth", in which a band of ice-free, or ice-thin, waters remains around the , allowing for a continued . This hypothesis appeals to scientists who observe certain features of the sedimentary record that can only be formed under open water, or rapidly moving ice (which would require somewhere ice-free to move to). Recent research observed geochemical cyclicity in , showing that the "snowball" periods were punctuated by warm spells, similar to cycles in recent Earth history. Attempts to construct computer models of a snowball Earth have also struggled to accommodate global ice cover without fundamental changes in the laws and constants which govern the planet. A less extreme snowball Earth hypothesis involves continually evolving continental configurations and changes in ocean circulation. Synthesised evidence has produced models indicating a "slushball Earth", where the stratigraphic record does not permit postulating complete global glaciations. Kirschivink's original hypothesis had recognised that warm tropical puddles would be expected to exist in a snowball earth. The snowball Earth hypothesis does not explain the alternation of glacial and interglacial events, nor the oscillation of glacial sheet margins. Scientific dispute The argument against the hypothesis is evidence of fluctuation in ice cover and melting during "snowball Earth" deposits. Evidence for such melting comes from evidence of glacial dropstones, geochemical evidence of climate cyclicity, and interbedded glacial and shallow marine sediments. A longer record from Oman, constrained to 13°N, covers the period from 712 to 545 million years ago—a time span containing the glaciations—and shows both glacial and ice-free deposition. There have been difficulties in recreating a snowball Earth with s. Simple GCMs with mixed-layer oceans can be made to freeze to the equator; a more sophisticated model with a full dynamic ocean (though only a primitive sea ice model) failed to form sea ice to the equator. In addition, the levels of necessary to melt a global ice cover have been calculated to be 130,000 ppm, which is considered by to be unreasonably large. Strontium isotopic data have been found to be at odds with proposed snowball Earth models of silicate weathering shutdown during glaciation and rapid rates immediately post-glaciation. Therefore, methane release from permafrost during was proposed to be the source of the large measured carbon excursion in the time immediately after glaciation. "Zipper rift" hypothesis Nick Eyles suggest that the Neoproterozoic Snowball Earth was in fact no different from any other glaciation in Earth's history, and that efforts to find a single cause are likely to end in failure. The "Zipper rift" hypothesis proposes two pulses of continental "unzipping"—first, the breakup of the supercontinent Rodinia, forming the proto-Pacific Ocean; then the splitting of the continent from , forming the proto-Atlantic—coincided with the glaciated periods. The associated tectonic uplift would form high plateaus, just as the is responsible for high topography; this high ground could then host glaciers. Banded iron formations have been taken as unavoidable evidence for global ice cover, since they require dissolved iron ions and anoxic waters to form; however, the limited extent of the Neoproterozoic banded iron deposits means that they may not have formed in frozen oceans, but instead in inland seas. Such seas can experience a wide range of chemistries; high rates of evaporation could concentrate iron ions, and a periodic lack of circulation could allow anoxic bottom water to form. Continental rifting, with associated subsidence, tends to produce such landlocked water bodies. This rifting, and associated subsidence, would produce the space for the fast deposition of sediments, negating the need for an immense and rapid melting to raise the global sea levels. High-obliquity hypothesis A competing hypothesis to explain the presence of ice on the equatorial continents was that Earth's was quite high, in the vicinity of 60°, which would place Earth's land in high "latitudes", although supporting evidence is scarce. A less extreme possibility would be that it was merely Earth's that wandered to this inclination, as the magnetic readings which suggested ice-filled continents depend on the magnetic and rotational poles being relatively similar. In either of these two situations, the freeze would be limited to relatively small areas, as is the case today; severe changes to Earth's climate are not necessary. Inertial interchange true polar wander The evidence for low-latitude glacial deposits during the supposed snowball Earth episodes has been reinterpreted via the concept of inertial interchange (IITPW). This hypothesis, created to explain palaeomagnetic data, suggests that Earth's axis of rotation shifted one or more times during the general time-frame attributed to snowball Earth. This could feasibly produce the same distribution of glacial deposits without requiring any of them to have been deposited at equatorial latitude. While the physics behind the proposition is sound, the removal of one flawed data point from the original study rendered the application of the concept in these circumstances unwarranted. Several alternative explanations for the evidence have been proposed. Survival of life through frozen periods , a type of hydrothermal vent}} A tremendous glaciation would curtail photosynthetic life on Earth, thus depleting atmospheric oxygen, and thereby allowing non-oxidized iron-rich rocks to form. Detractors argue that this kind of glaciation would have made life extinct entirely. However, microfossils such as s and s prove that, in shallow marine environments at least, life did not suffer any perturbation. Instead life developed a trophic complexity and survived the cold period unscathed. Proponents counter that it may have been possible for life to survive in these ways: * In reservoirs of and low-oxygen life powered by chemicals in deep oceanic s surviving in Earth's deep oceans and ; but would not have been possible there. * Under the ice layer, in (mineral-metabolizing) s theoretically resembling those in existence in modern glacier beds, high-alpine and Arctic talus permafrost, and basal glacial ice. This is especially plausible in areas of or activity. * In pockets of liquid water within and under the ice caps, similar to in Antarctica. In theory, this system may resemble communities living in the perennially frozen lakes of the Antarctic dry valleys. Photosynthesis can occur under ice up to 100 m thick, and at the temperatures predicted by models equatorial would prevent equatorial ice thickness from exceeding 10 m. * As eggs and dormant cells and spores deep-frozen into ice during the most severe phases of the frozen period. * In small regions of open water in deep ocean regions preserving small quantities of life with access to light and for photosynthesizers (not multicellular plants, which did not yet exist) to generate traces of oxygen that were enough to sustain some oxygen-dependent organisms. This would happen even if the sea froze over completely, if small parts of the ice were thin enough to admit light. These small open water regions may have occurred in deep ocean regions far from the or its remnants as it broke apart and drifted on the . * In layers of "dirty ice" on top of the ice sheet covering shallow seas below. Animals and mud from the sea would be frozen into the base of the ice and gradually concentrate on the top as the ice above evaporates. Small ponds of water would teem with life thanks to the flow of nutrients through the ice. Such environments may have covered approximately 12 per cent of the global surface area. * In small oases of liquid water, as would be found near resembling today. * In areas in the , where daytime tropical sun or volcanic heat heated bare rock sheltered from cold wind and made small temporary melt pools, which would freeze at sunset. However, organisms and ecosystems, as far as it can be determined by the fossil record, do not appear to have undergone the significant change that would be expected by a . With the advent of more precise dating, a phytoplankton extinction event which had been associated with snowball Earth was shown to precede glaciations by 16 million years. Even if life were to cling on in all the ecological refuges listed above, a whole-Earth glaciation would result in a biota with a noticeably different diversity and composition. This change in diversity and composition has not yet been observed—in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth. Implications A snowball Earth has profound implications in the history of on Earth. While many have been postulated, global ice cover would certainly have ravaged s dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials. Because about half of the oceans' water was frozen solid as ice, the remaining water would be twice as salty as it is today, lowering its freezing point. When the ice sheet melted, it would cover the oceans with a layer of hot freshwater up to 2 kilometres thick. Only after the hot surface water mixed with the colder and deeper saltwater did the sea return to a warmer and less salty state. The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the period. Effect on early evolution costata'', an organism of unknown affinity, with a quilted appearance}} The was a time of remarkable diversification of multicellular organisms, including animals. Organism size and complexity increased considerably after the end of the snowball glaciations. This development of multicellular organisms may have been the result of increased evolutionary pressures resulting from multiple cycles; in this sense, snowball Earth episodes may have "pumped" evolution. Alternatively, fluctuating nutrient levels and rising oxygen may have played a part. Another major glacial episode may have ended just a few million years before the . One hypothesis which has been gaining currency in recent years: that early snowball Earths did not so much affect the evolution of life on Earth as result from it. In fact the two hypotheses are not mutually exclusive. The idea is that Earth's life forms affect the global carbon cycle and so major evolutionary events alter the carbon cycle, redistributing carbon within various reservoirs within the biosphere system and in the process temporarily lowering the atmospheric (greenhouse) carbon reservoir until the revised biosphere system settled into a new state. The Snowball I episode (of the 2.4 to 2.1 billion years) and Snowball II (of the Precambrian's between 580–850 million years and which itself had a number of distinct episodes) are respectively thought to be caused by the evolution of and then the rise of more advanced multicellular animal life and life's colonization of the land. Effects on ocean circulation Global ice cover, if it existed, may—in concert with geothermal heating—have led to a lively, well mixed ocean with great vertical convective circulation. Occurrence and timing Neoproterozoic There were three or four significant ice ages during the late . Of these, the was the most significant, and the glaciations were also truly widespread. Even the leading snowball proponent Hoffman agrees that the 350 thousand-year-long Gaskiers glaciation did not lead to global glaciation, although it was probably as intense as the . The status of the "glaciation" or "cooling event" is currently unclear; some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe it may indeed be a third ice age. It was certainly less significant than the Sturtian or Marinoan glaciations, and probably not global in extent. Emerging evidence suggests that the Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis. Palaeoproterozoic The snowball Earth hypothesis has been invoked to explain glacial deposits in the of Canada, though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested. The glacial sediments of the Makganyene formation of South Africa are slightly younger than the Huronian glacial deposits (~2.25 billion years old) and were deposited at tropical latitudes. It has been proposed that rise of free oxygen that occurred during the removed methane in the atmosphere through oxidation. As the was notably weaker at the time, Earth's climate may have relied on methane, a powerful greenhouse gas, to maintain surface temperatures above freezing. In the absence of this methane greenhouse, temperatures plunged and a snowball event could have occurred. Karoo Ice Age Before the theory of continental drift, glacial deposits in strata in tropical continental areas such as India and South America led to speculation that the glaciation reached into the tropics. However, a continental reconstruction shows that ice was in fact constrained to the polar parts of the . References Category:Earth